The results of a determination performed in two different laboratories can only be compared if the techniques used are comparable, independently of the methodology or manufacturer. For this purpose, methods must be traceable to common materials and methods. Despite the common sense of this statement, only few laboratory determinations fulfill this requisite. In general, during the follow up of a patient, the physician must try to use the same laboratory or at least laboratories that use the same method. Moreover, laboratories should define the method used for determinations in their reports as indispensable information for attending physicians.